A metamorphic rock supplied to be some other form of rock, however it was readjusted inside the planet to end up being a new form of rock. The word metamorphism comes from ancient Greek words because that “change” (meta) and “form” (morph). The type of rock that a metamorphic rock provided to be, prior to metamorphism, is called the protolith. During metamorphism the mineral content and texture the the protolith are readjusted due to alters in the physical and chemical setting of the rock. Metamorphism deserve to be led to by burial, tectonic stress, heating by magma, or alteration by fluids. At progressed stages of metamorphism, the is typical for a metamorphic absent to construct such a different set of minerals and also such a thoroughly changed texture the it is complicated to identify what the protolith was.

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A absent undergoing metamorphism stays a heavy rock during the process. Rocks perform not melt during most conditions of metamorphism. At the highest possible grade that metamorphism, rocks begin to partly melt, in ~ which point the boundary of metamorphic conditions is surpassed and the igneous component of the absent cycle is entered.

Even despite rocks remain solid during metamorphism, fluid is generally existing in the microscope spaces between the minerals. This fluid phase may play a major role in the chemical reactions the are crucial part of exactly how metamorphism occurs. The liquid usually is composed largely that water.

Metamorphic rocks carry out a document of the processes that occurred inside planet as the rock to be subjected to transforming physical and chemical conditions. This provides the geologist literally “inside information” on what occurs within the Earth throughout such processes as the development of new mountain ranges, the collision the continents, the subduction the oceanic plates, and also the circulation of sea water into hot oceanic crust. Metamorphic rocks are favor probes that have gone down into the Earth and also come back, pass an document of the problems they encountered on your journey in the depth of the Earth.

Figure 1. The platy class in this huge outcrop that metamorphic rock present the effects of push on rocks during metamorphism.

In the big outcrop of metamorphic rocks in number 1, the rocks’ platy figure is a an outcome of the procedure metamorphism. Metamorphism is the addition of warm and/or press to present rocks, which reasons them to readjust physically and/or chemically so that they come to be a new rock. Metamorphic rocks may readjust so lot that they might not resemble the original rock.


Any form of rock—igneous, sedimentary, or metamorphic—can become a metamorphic rock. All the is needed is sufficient heat and/or push to transform the existing rock’s physics or chemistry makeup without melt the absent entirely.

Figure 2. A foliated metamorphic rock.

Rocks readjust during metamorphism because the minerals have to be stable under the new temperature and also pressure conditions. The need for security may cause the structure of minerals to rearrange and kind new minerals. Ions might move in between minerals to develop minerals of different chemical composition. Hornfels, v its alternative bands that dark and light crystals, is a good example of just how minerals rearrange themselves throughout metamorphism. Hornfels is shown in table 1.

Extreme push may likewise lead to foliation, the level layers that form in rocks as the rocks space squeezed by press (figure 2). Foliation normally develops when push is exerted in just one direction. Metamorphic rocks may additionally be non-foliated. Quartzite and limestone, shown in table 6, room nonfoliated.

The two main species of metamorphism room both connected to heat within Earth:

Regional metamorphism: alters in enormous quantities the rock over a wide area brought about by the too much pressure native overlying rock or indigenous compression brought about by geologic processes. Deep funeral exposes the absent to high temperatures.Contact metamorphism: transforms in a rock that is in contact with magma since of the magma’s too much heat.

Factors that control Metamorphism

The reason rocks undergo metamorphism is that the minerals in a rock are just stable under a restricted range of pressure, temperature, and chemical conditions. As soon as rocks room subjected to large enough transforms in this factors, the minerals will certainly undergo chemistry reactions that an outcome in their replacement by brand-new minerals, minerals that are stable in the new conditions.

Chemical ingredient of the Protolith

The type of rock undergoing metamorphism is a major factor in determining what kind of metamorphic absent it becomes. In brief the determine of the protolith theatre a huge role in the identity of the metamorphic rock. A fluid phase may introduce or remove chemical substances right into or the end of the rock throughout metamorphism, but in most metamorphic rock, many of the atom in the protolith will certainly be current in the metamorphic rock after metamorphism; the atoms will most likely be rearranged into new mineral forms within the rock. Therefore, not only does the protolith recognize the early stage chemistry the the metamorphic rock, many metamorphic rocks perform not readjust their mass (overall) chemistry compositions an extremely much throughout metamorphism. The fact that many metamorphic rocks retain many of their initial atoms way that also if the rock to be so extensively metamorphosed the it no longer looks in ~ all like the protolith, the rock can be analyzed in regards to its mass chemical ingredient to identify what kind of rock the protolith was.


Temperature is another significant factor the metamorphism. There space two means to think around how the temperature the a rock can be raised as a result of geologic processes.

If rocks are hidden within the Earth, the deeper castle go, the higher the temperature they experience. This is because temperature inside the planet increases along what is dubbed the geothermal gradient, or geotherm for short. Therefore, if rocks are simply buried deep sufficient enough sediment, lock will experience temperatures high enough to reason metamorphism. This temperature is around 200ºC (approximately 400ºF).

Tectonic procedures are another method rocks can be moved deeper follow me the geotherm. Faulting and also folding the rocks the the crust, can move rocks to much higher depth than simple burial can.

Yet another way a absent in the earth crust have the right to have the temperature considerably increased is by the intrusion the magma nearby. Magma intrusion subjects adjacent rock to greater temperature through no increase in depth or pressure.


Pressure is a measure up of the stress, the physical force, being used to the surface of a material. That is defined as the force per unit area exhilaration on the surface, in a direction perpendicular come the surface.

Lithostatic push is the pressure exerted ~ above a absent by all the neighboring rock. The source of the press is the load of every the rocks above. Lithostatic pressure boosts as depth within the earth increases and is a uniform stress—the pressure applies equally in all directions ~ above the rock.

If press does not use equally in all directions, differential anxiety occurs. There room two types of differential stress.

Normal stress compresses (pushes together) rock in one direction, the direction of best stress. In ~ the same time, in a perpendicular direction, the absent undergoes tension (stretching), in the direction of minimum stress.

Shear tension pushes one next of the absent in a direction parallel to the side, while at the very same time, the other side the the rock is being moved in opposing direction.

Differential stress has a major influence top top the appearance of a metamorphic rock. Differential stress can flatten pre-existing grains in the rock, as displayed in the diagram below.


Metamorphic mineral that flourish under differential tension will have a desired orientation if the minerals have actually atomic structures that have tendency to make them form either flat or elongate crystals. This will be especially evident for micas or various other sheet silicates that grow during metamorphism, such together biotite, muscovite, chlorite, talc, or serpentine. If any type of of these flat minerals are cultivation under regular stress, lock will grow with their sheets oriented perpendicular to the direction of maximum compression. This results in a absent that can be easily broken along the parallel mineral sheets. Together a absent is said to be foliated, or to have foliation.



Any open room between the mineral grains in a rock, but microscopic, might contain a fluid phase. Most commonly, if there is a liquid phase in a rock throughout metamorphism, it will be a hydrous fluid, consists of water and also things liquified in the water. Less commonly, it may be a carbon dioxide fluid or some various other fluid. The presence of a fluid phase is a significant factor during metamorphism since it helps recognize which metamorphic reactions will certainly occur and also how quick they will certainly occur. The fluid phase can additionally influence the price at i m sorry mineral crystals deform or readjust shape. Most of this affect is as result of the dissolved ions that pass in and also out that the fluid phase. If throughout metamorphism sufficient ions are presented to or eliminated from the absent via the liquid to adjust the bulk chemical composition of the rock, the absent is said to have actually undergone metasomatism. However, most metamorphic rocks execute not experience sufficient change in their bulk chemistry to be considered metasomatic rocks.


Most metamorphism that rocks bring away place progressively inside the Earth. Local metamorphism takes location on a timescale of countless years. Metamorphism usually requires slow transforms to rocks in the hard state, as atoms or ion diffuse the end of turbulent minerals that are breaking under in the offered pressure and temperature conditions and also migrate into brand-new minerals that space stable in those conditions. This form of chemistry reaction take away a long time.

Grades of Metamorphism

Metamorphic grade refers to the general temperature and pressure conditions that prevailed throughout metamorphism. As the pressure and also temperature increase, rocks undergo metamorphism at greater metamorphic grade. Rocks changing from one kind of metamorphic rock to one more as castle encounter higher grades that metamorphism are claimed to be experience prograde metamorphism.

Low-grade metamorphism takes location at approximately 200–320 ºC and reasonably low pressure. This is no far beyond the problems in i beg your pardon sediments obtain lithified into sedimentary rocks, and also it is typical for a low-grade metamorphic rock to watch somewhat favor its protolith. Low grade metamorphic rocks often tend to characterized by an abundance of hydrous minerals, minerals that contain water within their decision structure. Instances of low grade hydrous minerals include clay, serpentine, and chlorite. Under low grade metamorphism many of the metamorphic minerals will not grow large enough come be viewed without a microscope.

Medium-grade metamorphism takes place at approximately at 320–450 ºC and also at center pressures. Short grade hydrous mineral are replaced by micas such together biotite and muscovite, and also non-hydrous minerals such together garnet may grow. Garnet is an example of a mineral i beg your pardon may type porphyroblasts, metamorphic mineral grains that are bigger in size and much more equant in form (about the same diameter in all directions), therefore standing out amongst the smaller, flatter, or an ext elongate minerals.

High-grade metamorphism takes place at temperature above about 450 ºC. Micas tend to rest down. Brand-new minerals such together hornblende will certainly form, which is stable at greater temperatures. However, together metamorphic grade boosts to even higher grade, all hydrous minerals, which consists of hornblende, may failure and be replaced by other, higher-temperature, non-hydrous minerals such together pyroxene.

Index Minerals

Index minerals, i m sorry are signs of metamorphic grade. In a offered rock type, i m sorry starts with a details chemical composition, lower-grade index minerals are changed by higher-grade table of contents minerals in a sequence of chemical reactions the proceeds as the rock undergoes prograde metamorphism. Because that example, in rocks made of metamorphosed shale, metamorphism may prograde through the following index minerals:

chlorite characterizes the lowest local metamorphic gradebiotite replace instead instead chlorite in ~ the next metamorphic grade, which could be taken into consideration medium-low gradegarnet appears at the next metamorphic grade, tool gradestaurolite marks the next metamorphic grade, i beg your pardon is medium-high gradesillimanite is a properties mineral of high grade metamorphic rocks

Index minerals are used by geologists to map metamorphic great in regions of metamorphic rock. A geologist maps and collects absent samples across the an ar and clues the geologic map with the place of each rock sample and also the type of index mineral that contains. By drawing lines about the areas where each form of index mineral occurs, the geologist delineates the zones of different metamorphic qualities in the region. The lines are well-known as isograds.

Types the Metamorphism

Regional Metamorphism

Regional metamorphism wake up where big areas of rock room subjected to huge amounts that differential anxiety for long intervals of time, problems typically connected with hill building. Mountain structure occurs at subduction zones and also at continental collision area where two plates every bearing continent crust, converge upon every other.

Most foliated metamorphic rocks—slate, phyllite, schist, and gneiss—are developed during regional metamorphism. As the rocks end up being heated at depth in the earth during regional metamorphism they end up being ductile, which means they are fairly soft also though they space still solid. The folding and also deformation that the absent while it is ductile may significantly distort the original shapes and orientations the the rock, creating folded layers and also mineral veins that have highly deformed or even convoluted shapes. The diagram below shows folds developing during an early stage of local metamorphism, follow me with breakthrough of foliation, in solution to typical stress.

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The photograph listed below shows high-grade metamorphic absent that has undergone several stages of foliation breakthrough and folding during local metamorphism, and also may also have reached such a high temperature the it started to melt.